Genetic risk can be conferred by subtle differences in individual genomes within a population. Genes can differ between individuals due to genomic variability, the most frequent of which are due to single nucleotide polymorphisms (SNPs). SNPs can be located, on average, every 500-1000 base pairs in the human genome. Additional genetic polymorphisms in a human genome can be caused by duplication, insertion, deletion, translocation and/or inversion, of short and/or long stretches of DNA. Thus, in general, genetic variability among individuals occurs on many scales, ranging from single nucleotide changes, to gross changes in chromosome structure and function. Recently, many copy number variations (CNVs) of DNA segments, including deletions, insertions, duplications, amplifications, and complex multi-site variants, ranging in length from kilobases to megabases in size, have been discovered (Redon, R. et al. Nature 444:444-54 (2006) and Estivill, X. & Armengol, L. PLoS Genetics 3(10): e190 (2007)). To date, known CNVs account for over 15% of the assembled human genome (Estivill, X. Armengol, L. PLoS Genetics 3(10): e190 (2007)). However, a majority of these variants are extremely rare and cover a small percentage of a human genome of any particular individual.
Parkinson's Disease (also known as Parkinson disease, Parkinson's, sporadic parkinsonism, primary parkinsonism, PD, or paralysis agitans) is a degenerative disorder of the central nervous system. Parkinson's disease (PD) can be characterized by a progressive degeneration of dopaminergic neurons in the midbrain. While PD is a complex disorder of unknown etiology, it is postulated that symptom manifestation occurs after the fraction of functional dopaminergic cells falls below a threshold of twenty percent. Symptoms of PD can include tremor, muscular rigidity, bradykinesia, akinesia, and postural instability. A hallmark of sporadic (also termed idiopathic) Parkinson's disease can be the progressive loss of dopaminergic neurons and a depletion of dopamine, more specifically in the basal ganglia, and is thought to result from a combination of genetic predisposition (Vaughn, J. R., et al., 2001, Ann. Hum. Genet. 65:111; Farrer M. J., 2006, Nat. Rev. Genet. 7:306) and environmental factors (Shapira, A. H., 2001, Adv. Neurol. 86:155; Obeso J. A., et al., 2010, Nat. Med. 16:653). Thus, research efforts have focused on discovering means to prevent, protect and restore the dopaminergic cell network (Latchman, D. S., et al., 2001 Rev. Neurosci. 12:69). As genetic polymorphisms/variants conferring risk in neurological diseases, including PD, are uncovered, genetic testing can play a role for clinical therapeutics.
Despite these advances towards an understanding of the etiology of neurological disorders, a large fraction of the genetic contribution to these disorders, for example, PD, remains undetermined Identification of underlying genetic variants that can contribute to neurological disorder pathogenesis can aid in the screening and identification of individuals at risk of developing these disorders and can be useful in a diagnostic setting and for disease management. There is a need to identify new treatments for neurological diseases, such as PD, and the identification of novel genetic risk factors or disease-causing genetic variants can assist in the development of potential therapeutics and agents. There is also a need for improved assays for predicting and determining potential treatments and their effectiveness.